I was curious about this injury, because it seems so easy for someone to injure themselves this way, and I see from a google search that an alarming number of people have had exactly this accident.
I also see some crossbow manufacturers have safety switches you have to hold down with the thumb of your supporting hand before it will fire. The switch is located well out of the path of the amputating string. That's a smart idea. I bet all of them will have that in a few years.
I also read about a few different ways you can fuck yourself up real good with a crossbow. It sounds pretty dangerous if you don't read up about it and pay really close attention to what you are doing. Be careful!
(Like if it slips out from under your foot when you are cocking it, and it's under a lot of tension, the whole thing can spring back into your face and break a nose or take an eye
.)